Answer: Following Texas' successful war of independence against Mexico in 1836, President Martin van Buren refrained from annexing Texas after the Mexicans threatened war. ... His efforts culminated on April 12 in a Treaty of Annexation, an event that caused Mexico to sever diplomatic relations with United States. Polk defeated Whig Henry Clay in a close contest turning on the controversial issues of slavery and the annexation of the Republic of Texas. President John Tyler's pursuit of Texas annexation threatened the unity of both major parties.